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Electrical Load Calculation: NEC Article 220
Why This Matters
Every residential service upgrade, new construction permit, and subpanel installation requires a load calculation. The inspector wants to see the math proving your 200-amp panel can carry the load. NEC Article 220 provides two methods: standard and optional. Both get you to the same answer, but the optional method is faster and usually yields a smaller number.
This is different from wire sizing and voltage drop. Those size individual circuits. This sizes the whole service.
Standard Method (NEC 220.40-220.82)
Add up every load in the house, apply demand factors, and divide by voltage to get amperage.
Step 1: General Lighting and Receptacles
NEC Table 220.12 assigns 3 VA per square foot of living space. For a 2,400 sq ft house:
2,400 x 3 = 7,200 VA
This covers all general lighting outlets and most receptacle outlets.
Step 2: Small Appliance and Laundry Circuits
NEC 220.52 requires at least two 20-amp small appliance branch circuits for the kitchen and dining area, plus one 20-amp laundry circuit.
2 kitchen circuits x 1,500 VA = 3,000 VA 1 laundry circuit x 1,500 VA = 1,500 VA
Total: 4,500 VA
Step 3: Apply Demand Factor to Steps 1+2
Add general lighting and small appliance loads together: 7,200 + 4,500 = 11,700 VA.
NEC Table 220.42 demand factor:
- First 3,000 VA at 100% = 3,000 VA
- Remaining 8,700 VA at 35% = 3,045 VA
Derated total: 6,045 VA
Step 4: Fixed Appliances
List every fixed appliance with its nameplate rating. Common ones:
- Dishwasher: 1,500 VA
- Garbage disposal: 960 VA (8 amps x 120V)
- Bathroom exhaust fans (total): 240 VA
- Garage door opener: 600 VA
- Microwave (if hardwired): 1,500 VA
NEC 220.53: if you have four or more fixed appliances (other than range, dryer, HVAC, water heater), you can apply a 75% demand factor.
Five fixed appliances totaling 4,800 VA x 0.75 = 3,600 VA
Step 5: Range (Cooktop and Oven)
NEC Table 220.55 for household cooking equipment. A single range rated up to 12 kW: demand is 8 kW (8,000 VA).
If the range is rated over 12 kW, you add 5% of the nameplate for each kW above 12. A 14.4 kW range: 8,000 + (2.4 x 400) = 8,960 VA.
Step 6: Dryer
NEC 220.54: minimum 5,000 VA or nameplate rating, whichever is larger. Most residential dryers: 5,000 VA.
Step 7: HVAC
NEC 220.82(C)(1-6) says use the largest of: heating load or cooling load. You don’t add them because they don’t run at the same time (in theory).
Central AC: 5,000 VA (typical 3-ton unit compressor) Electric furnace: 10,000 VA Heat pump: 6,000 VA
If the house has a heat pump with electric backup strips, you add the compressor plus the backup heat since they can run simultaneously. That’s the gotcha that catches people. A heat pump with 10 kW backup strips: 6,000 + 10,000 = 16,000 VA for HVAC.
Step 8: Water Heater
Standard 50-gallon electric: 4,500 VA. Tankless electric: 18,000-36,000 VA. A tankless water heater can single-handedly push a house past 200-amp service. This is why a lot of electricians steer clients toward gas tankless or recommend a panel upgrade alongside the installation.
Step 9: Total It Up
For our 2,400 sq ft house with all-electric appliances:
| Load | VA |
|---|---|
| General lighting + small appliance (derated) | 6,045 |
| Fixed appliances (75% demand) | 3,600 |
| Range (8 kW) | 8,000 |
| Dryer | 5,000 |
| Heat pump + backup | 16,000 |
| Water heater (standard) | 4,500 |
| Total | 43,145 VA |
Service amperage: 43,145 / 240V = 179.8 amps.
That house needs 200-amp service. A 150-amp panel wouldn’t pass inspection.
Optional Method (NEC 220.82)
Faster. Often gives a lower number because it applies a blanket demand factor.
Step 1: General Loads at 100%
Add everything: general lighting (3 VA/sq ft x 2,400 = 7,200), small appliance circuits (4,500), all nameplate loads for all appliances. Total everything at full nameplate.
Say the total is 56,000 VA.
Step 2: Apply Demand Factor
NEC 220.82(B): first 10,000 VA at 100%, remainder at 40%.
10,000 VA at 100% = 10,000 46,000 VA at 40% = 18,400
Total: 28,400 VA
Step 3: Add HVAC
Add the largest motor and the HVAC load at 100%.
Heat pump + backup: 16,000 VA Largest motor (AC compressor): included above
Total: 28,400 + 16,000 = 44,400 VA
Service amperage: 44,400 / 240 = 185 amps. Still 200-amp service, but notice the optional method gave a slightly different path to the same conclusion.
When 200 Amps Isn’t Enough
EV chargers changed the math. A Level 2 charger pulls 7,680 VA (32 amps at 240V) to 11,520 VA (48 amps). Add that to our 43,145 VA house and you’re at 50,825 to 54,665 VA. Still under 200 amps (228 amps max), but getting tight. Two EVs and a tankless water heater can push a house past 200-amp service into 320-amp or 400-amp territory.
NEC 2023 added Article 625.42 load management provisions for EV chargers. An energy management system that limits simultaneous charging and HVAC can reduce the calculated load and keep you within 200 amps. Check with your local AHJ (authority having jurisdiction) for adoption status — not all jurisdictions have adopted the 2023 code yet.
The Quick Way
SiteCalc has a service load calculator based on NEC Article 220. Plug in the square footage, check off your appliances, enter nameplate ratings, and it runs both the standard and optional methods. It shows which method yields the lower number and gives you the minimum service size. The code references feature shows the specific NEC table citations for each demand factor. Faster than a spreadsheet and less error-prone than doing it on paper during a panel swap.